1. Field of the Invention
This invention relates to an automatic transmission control system.
2. Description of the Prior Art
An automatic transmission for a vehicle generally comprises a torque converter and a transmission gear mechanism such as a planetary-gear train as two basic parts. In the automatic transmission, gear-shifting is generally done by a hydraulic mechanism and the hydraulic circuit is switched by solenoid valves so that the frictional elements such as a brake or a clutch are properly operated to change the power train to shift the transmission to a desired gear. In order to switch the hydraulic circuit by the solenoid valves, that the running state of the vehicle has crossed a predetermined gear-shifting property line is generally detected by an electronic control device and the solenoid valves are selectively operated according to an upshifting signal or a downshifting signal output from the electronic control device.
In such automatic transmissions, whether or not gear-shifting is completed must be sometimes detected. That is, since slip of the torque converter cannot be avoided, there is generally provided a lockup mechanism for directly connecting the engine output shaft and the output shaft of the torque converter for the purpose of improving fuel economy, for instance. The lockup mechanism is applied and released by controlling the hydraulic pressure fed to the fluid actuators associated with the lockup mechanism according to a lockup signal or a lockup-release signal output from the electronic control device on the basis of predetermined lockup control characteristics.
If gear-shifting is effected while the lockup mechanism is applied, a large shock occurs. Therefore, as disclosed in U.S. Pat. No. 4,393,467, the lockup mechanism is generally released when the gear-shifting is to be effected so that torque fluctuation generated upon the gear-shifting is absorbed by the torque converter. Further, when downshift takes place, the driving load of the frictional engagement elements is increased, and the line pressure for engaging the engagement elements must be increased. The line pressure must be increased (backup) after completion of the downshift in order to avoid gear-shifting shock. Therefore, whether or not the gear-shifting is completed must be accurately detected.
Conventionally, this detection has been effected by detecting the rate of change in rotational speed of the torque converter based on the fact that the rate of change in rotational speed of the torque converter is large during gear-shifting as disclosed in U.S. Pat. No. 4,468,988. That is, when the rate of change is reduced below a predetermined value, it is determined that the gear-shifting has been completed.
However, the rate of change in rotational speed of the torque converter does not always change linearly. For example, though the rotational speed of the output shaft of the torque converter tends to be reduced by a large amount upon upshifting, the reduction is not always linear with sometimes the rotational speed being further reduced by a large amount after the reducing tendency has weakened. Therefore, if detection of whether the gear-shifting is completed depends solely upon the rate of change in the rotational speed of the torque converter, there is a possibility that it will be determined that gear-sifting is completed when the reducing tendency is weakened. If said predetermined value is minimized to avoid such problems, it becomes very difficult to distinguish change in rotational speed of the torque converter during the gear-shifting from that during other operating conditions.